Nile tilapia (Oreochromis niloticus) was injected intraperitoneally with types A, D and A+D of Clostridium perfringcns under .controlled laboratory conditions. The biochemical and haematologicai analyses were carried out on the 3rd, 7lJl and I4lh days post injection. The obtained results showed that:1. Clinically, the infected fish showed loss of appetite, sluggish and erratic swimming, loss of equilibrium and listless. At the site of injection, there was brownish caseous material which easily separated from the muscle. Gills were congested. Internally, the organs were friable and had a generalized hyperemic appearance.2. Histopathologically. the striated muscle, at the site of injection showed edema and necrosis with leucocytic infiltration. The gills showed sloughing of secondary lamellae and desquamation of the epithelial covering of secondary lamellae.3. Significantly decreased numbers of erythrocytes were detected after injection with C perfringens type A or D at all periods. Sporadically significant decreases in haemoglobin and haematocrit were found. Other haematologicai indices such as mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC) revealed inconsistent fluctuations.4. The leucocytes were decreased significantly in fish infected with C perfringens types D and A+D on the 7th and 14lh days respectively.5. Plasma and muscle total proteins showed a significant decrease whereas liver total proteins recorded a significant increase in comparison with the corresponding control values.6. Plasma glucose was significantly increased in fish injected with C perfringens type D at all periods, but significantly decreased with the combination of A+D.7. Plasma total lipids were significantly decreased in fishes injected with C perfringens type D as well as with the combination of A+D.8. Plasma alkaline phosphatase activity significantly decreased in all groups at al] periods studied.9. A genera] tendency of a significant decrease was observed in plasma aspartate aminotransferase (AST) activity paralleled with an increase in alanine aminotransferase (ALT) activity after injection with the two strains of C perfringens or their combination. There were some changes following this trend with respect to liver AST and muscle ALT.In* conclusion. Clostridium perfringens bacteria caused a high mortality rate amone. the cultured fish, while the drastic effect of the mild bacteria infection was inflected more on the fish physiological status and as a consequence decreases the farm production. Either mild or severe bacteria! infection should receive an immediate eye of inspection.